Use a doubly-linked list for _IO_list_all (bug 27777)

This patch fixes BZ #27777 "fclose does a linear search, takes ages when
many FILE* are opened".  Simply put, the master list of opened (FILE*),
namely _IO_list_all, is a singly-linked list.  As a consequence, the
removal of a single element is in O(N), which cripples the performance
of fclose().  The patch switches to a doubly-linked list, yielding O(1)
removal.  The one padding field in struct _IO_FILE, __pad5, is renamed
to _prevchain for a doubly-linked list.  Since fields in struct _IO_FILE
after the _lock field are internal to glibc and opaque to applications.
We can change them as long as the size of struct _IO_FILE is unchanged,
which is checked as the part of glibc ABI with sizes of _IO_2_1_stdin_,
_IO_2_1_stdout_ and _IO_2_1_stderr_.

NB: When _IO_vtable_offset (fp) == 0, copy relocation will cover the
whole struct _IO_FILE.  Otherwise, only fields up to the _lock field
will be copied to applications at run-time.  It is used to check if
the _prevchain field can be safely accessed.

After opening 2 million (FILE*), the fclose() of 100 of them takes quite
a few seconds without the patch, and under 2 seconds with it on a loaded
machine.

No test is added since there are no functional changes.

Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Alexandre Ferrieux <alexandre.ferrieux@orange.com>
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>